Hydraulic control system for an automatic transmission for a motor vehicle

ABSTRACT

A hydraulic control system for an automatic transmission, having a torque converter or a hydraulic coupler and a servo-operated speed change gear, for use in vehicles, includes a pressure regulator regulating a control hydraulic pressure supplied to the servo-operated speed change gear, and further includes a pressure modulator valve connected to the pressure regulator to assure smooth torque transmission for a large torque demand during low speed operation of the vehicle. A compensating pressure provided by the pressure modulator valve is applied to the pressure regulator valve to assure a control pressure, proportional to the throttle pressure opening or the torque demand, being applied to the servo-operated speed change gear. The pressure modulator valve restricts the control pressure to a value below the maximum constant pressure for a throttle valve opening exceeding a certain value, preventing excessive increase in the control pressure and assuring a constant low control pressure when the vehicle speed increases to a predetermined speed or when the transmission is set for the usual driving range.

United States Patent Chnnma [54] HYDRAULIC CONTROL SYSTEM FOR ANAUTOMATIC TRANSMISSION FOR A MOTOR VEHICLE [72] inventor: KiyoshiOhnuma, Toyoda-shi, Japan Toyota Jidosha Kogyo Kabushiki Kaisha,Toyoda-shi, Japan 221 Filed: Dec. 17,1969

211 Appl.No.: 885,849

[73] Assignee:

[ 1 Feh.22,i972

Primary ExaminerArthur T. McKeon Attorney-McGlcw and Toren [57] ABSTRACTA hydraulic control system for an automatic transmission, having atorque converter or a hydraulic coupler and a servooperated speed changegear, for use in vehicles, includes a pressure regulator regulating acontrol hydraulic pressure supplied to the servo-operated speed changegear, and further includes a pressure modulator valve connected to thepressure regulator to assure smooth torque transmission for a largetorque demand during low speed operation of the vehicle. A compensatingpressure provided by the pressure modulator valve is applied to thepressure regulator valve to assure a control pressure, proportional tothe throttle pressure opening or the torque demand, being applied to theservo-operated speed change gear. The pressure modulator valve restrictsthe control pressure to a value below the maximum constant pressure fora throttle valve opening exceeding a certain value, preventing excessiveincrease in the control pressure and assuring a constant low controlpressure when the vehicle speed increases to a predetermined speed orwhen the transmission is set for the usual driving range.

4 Claims, 5 Drawing Figures PATENTEDFEBZZ 1972 3,643 527 sum 1 OF 3 FIG.4

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SUMMARY OF THE INVENTION This invention relates to hydraulic pressurecontrol systems for automatic transmissions for use in motor vehiclesand, more particularly, to a novel hydraulic pressure control systemincluding a pressure modulator valve in controlling relation with apressure regulator valve and operable to assure adequately high controlhydraulic pressure at low vehicle speeds while preventing excessivehydraulic control pressure at higher vehicle speeds.

An arrangement in accordance with the present invention results insmooth torque transition when the vehicle is climbing a grade or is putin motion on an upgrade. When the vehicle speed increases to apredetermined value, coordinated with a throttle opening, the controlpressure is lowered to a constant low pressure such as required forordinary running and. independently of the throttle opening, so as toeffect smooth automatic speed change. Furthermore, in the speed range inwhich the control hydraulic pressure is controlled in proportion to thetorque demand, when the throttle opening exceeds a certain value, thehydraulic control pressure is suppressed or limited to a certain maximumvalue so as not to become excessive. This protects the conduit system aswell as resulting in lowering of impact or noise during automatic gearchanging. Thereby, adequate operating pressure is assured without thenecessity for heavyweight equipment.

More particularly, in accordance with the present invention, thehydraulic control system includes a pressure modulator valve operable tosupply a compensating pressure to the pressure regulator valve usuallyincluded in an automatic transmission of the type having a torqueconverter or a fluid coupler and a hydraulic servo-operated gearchanger. The pressure regulator valve regulates the control hydraulicpressure supplied to the servos. The pressure modulator valve includes amovable valve body and a movable plug, which is movable independently ofthe valve body but which is capable of exerting an operating force onthe movable valve body.

A hydraulic pressure proportional to the engine throttle opening isapplied to the movable valve body, whereby a compensating pressurecorresponding to the throttle opening is applied to the pressureregulator valve to control the control hydraulic pressure. A hydraulicpressure proportional to the vehicle speed, and the control hydraulicpressure, are applied to the movable plug in opposition so as tocounteract each other to an extent such that the movable plug does notexert any operating force on the movable valve body as long as thevehicle speed is below a predetermined valve. When the vehicle speedequals or exceeds this predetermined value, so that the pressureproportional to the vehicle speed exceeds a predetermined value, themovable plug exerts an operating force on the movable valve body to movethe same in a direction such that a compensating pressure, which isindependent of the throttle opening, is applied to the pressureregulator valve to maintain the control hydraulic pressure at a constantvalue.

In a narrow speed range, the throttle opening is substantiallyproportional to the output torque, so that a pressure proportional tothe throttle opening is applicable in the low-speed range. When a higherspeed is desired by acceleration of the vehicle, through opening thethrottle, the control pressure corresponding to the throttle opening isno longer necessary, so that the control hydraulic pressure is decreasedto a certain constant value at a higher vehicle speed.

With the invention arrangement, as a control hydraulic pressure which isproportional to the throttle. opening is available, this pressure, foralarge opening of the throttle, will be correspondingly high, so that itwill be unnecessarilyhigh for the servos and would require an excessivestrength for the conduit system. Accordingly, in further accordance withthe invention, the compensating pressure is maintained constant, forexample, at a zero value, to prevent the control pressure from buildingup to a higher pressure than a desired maximum, between a certainopening and a full opening of the throttle valve. Thus, in the inventionhydraulic control system, the control hydraulic pressure has thecharacteristics required for the hydraulic servos in a special low-speedrange, so that a sufficient torque transmission can be performed whenthe vehicle is climbing a grade, starting up on a grade, etc.

An object of the invention is to provide an improved hydraulic pressurecontrol system for automatic transmissions for motor vehicles.

Another object of the invention is to provide such an improved hydrauliccontrol system in which a control hydraulic pressure, increased inaccordance with "the torque demand, is provided for operating the servosduring low-speed operation, with the control hydraulic pressure beingreduced to a constant level when the torque demand decreases, as inhigher speed operation of the vehicle.

A further object of the invention is to provide such a hydraulicpressure control system including a pressure regulating valve regulatingthe control hydraulic pressure, and a pressure modulating valveconnected in controlling relation with the pressure regulating valve.

Another object of the invention is to provide such an improved hydraulicpressure control system in which the pres sure modulating valve, atvehicle speeds up to a predetermined value, is operable in accordancewith the engine throttle opening to assure a compensating pressureadjusting the pressure regulating valve to provide a control hydraulicpressure also corresponding to the throttle opening.

Another object of the invention is to provide such a hydraulic pressurecontrol system in which, when the vehicle speed exceeds a predeterminedvalue, the pressure modulating valve operates independently of thethrottle opening to provide a compensating pressure to the pressureregulating valve maintaining the control hydraulic pressure at asubstantially constant value.

A further object of the invention is to provide such a hydraulicpressure control system in which, as compared with prior art systems,the weight and the cost of the automatic transmission are substantiallyreduced.

For an understanding of the principles of the invention, reference ismade to the following description of a typical embodiment thereof asillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a somewhat schematic and diagrammatic illustration of oneexample of an automatic transmission to which the present invention isapplicable;

FIG. 2 is a hydraulic circuit diagram for the automatic transmissionillustrated in FIG. 1;

FIG. 3 is a circuit diagram of a pressure regulator valve and anassociated pressure modulator valve, embodying the present invention, toa larger scale than in FIG. 2;

FIG. 4 is a graphical illustration of the characteristic curves ofvarious hydraulic pressures for the throttle openings, in accordancewith the pressure modulator valve of the invention; and

FIG. is a graphical illustration of characteristic curves of controlpressures for vehicle speeds in accordance with the hydraulic controlsystem of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a pumpimpeller 2 is connected directly to an engine output shaft 1, and thepower of the engine is transmitted to a turbine runner 3 by oil frompump impeller 2. The oil from turbine runner 3 is delivered to a stator4 from which it is again supplied to pump impeller 2. By virtue of suchoil flow or circulation, a torque is transmitted to a turbine shaft 5.Thus, pump impeller 2, turbine runner 3 and stator 4 constitute a torqueconverter. The torque from turbine shaft 5 is transmitted, at selectivespeeds, to an output shaft 16 through a speed change gear 20.

Speed change gear 20 comprises a speed changing unit including amultiple disc clutch 8 interposed between turbine shaft 5 and a drum 7.A brake band 21 is engageable with the external circumference of drum 7for arresting motion of the drum. The speed change gear further includesa planetary gear unit having an input sun gear 10 secured to rotate withturbine shaft 5, a low-speed sun gear 11 connected to rotate with drum7, planetary pinions 13 meshing with sun gear 11 and planetary pinions12, a carrier 14 rotatably supporting planetary pinions 12 and 13, aninternal ring gear 15 meshing with planetary pinions 13, and a multipledisc brake 22 provided on the external surface of internal ring gear 15.An output shaft 16 is connected to rotate with carrier 14.

To supply hydraulic fluid, such as oil, under pressure to the hydraulicservos of the torque converter, to clutch 8 and brake 22 and to brakeband 21, a front oil pump 51 is mounted on a boss which rotates withpump impeller 2, and a rear pump 52 is mounted on output shaft 16. Toobtain a pressure proportional to the vehicle speed, a governor 53 isalso provided on output shaft 16.

As is well known, such a hydraulic automatic transmission effects gearselection between two forward speeds and one reverse speed, by selectiveoperation of clutch 8 and brake 22 and brake band 21. More particularly,the first or low speed is obtained by releasing clutch 8 and brake 22and applying brake band 21 to drum 7 to hold the latter stationary, aswell as to hold stationary the low sun gear 11 interlocked with drum 7.Thereby, a low forward speed is provided which is lower than therotating speed of turbine shaft 5.

A second or higher forward speed is obtained by applying clutch 8,releasing brake band 21 and releasing brake 22, so that low-speed sungear 11 is rotated, together with input shaft 10, in the same directionto connect turbine shaft 5 directly with output shaft 16 through pinions12 and 13. This establishes a higher forward speed.

A reverse speed in provided by releasing clutch 8 and brake band 21, andby engaging brake 22 to restrain rotation of internal ring gear 15.Thereby, output shaft 16 is caused to rotate in the reverse direction ata reduced speed.

Gear or speed selection is effected by movement of a manual controlvalve 57in the hydraulic control circuit shown in FIG. 2, such movementbeing effected by a gear or speed selecting lever. Valve 57 is movedbetween low (L), drive (D), neutral" (N), reverse" (R) and parking" (P).When the selection lever is placed in position D, selection of the firstand second forward speeds, as mentioned above, is automatically effectedin dependence on the vehicle speed and the engine throttle opening,through the medium of a shift valve 59 connected to the engine throttle.

In the hydraulic circuit or control diagram of FIG. 2, the hydraulicservos of clutch 8 and brake 22 and of brake band 21 are illustrated atthe left, and the hydraulic fluid or oil under pressure is supplied tothese servos either directly,

through manually operated valve 57, or automatically, through shiftvalve 59, to effect the speed or gear changes as mentioned above. In theposition illustrated in the drawing, the manual valve 57 is at the driveposition D and gear changeover for the first and second forward speedsis automatically effected by alternately applying clutch 8 and brakeband 21.

The hydraulic fluid or oil for operating these servos is drawn from oilsumps 50a, 50b, by means of front oil pump 51 and rear oil pump 52. Thehigher of the oil pressures from these two pumps is selected byrespective check valves 55 and 56, for supply to control pressure line101. As illustrated in FIG. 2, oil or hydraulic fluid under pressure issupplied by pump 52 to line 101 through check valve 56. The pressure inline 101 is called a control pressure" or a line pressure."

One end of line 101 is connected to a chamber 203 of a pressureregulator valve 54, and the pressure in chamber 202 acts as a force tourge valve body 54a to the right against the bias of a spring 54b.Hydraulic fluid from line 101, in chamber 203, is discharged throughchamber 204 to sump 50a. The quantity of oil thus discharged increasesin accordance with an increase in the pressure in line 101, and lowersthe discharge pressure of pump 52. As a result, the control pressure inline 101 is maintained at a constant value corresponding to the bias orforce of spring 54b and irrespective of the operating speed of pump 52.

The regulated control pressure is supplied directly, from chamber 203 ofregulator valve 54 through line 101, to torque converter C, andsimultaneously, through a chamber 211 of manually controlled valve 57,to lines 103 and 111. The regulator control pressure is also applieddirectly to a chamber 221 of the servo for brake band 21, through lines111 and 112. This urges a piston 21a of the servo to the left, againstthe bias of a spring 21b, to apply brake band 21. Thereby the first orlow speed is established. I

In addition, and as explained in more detail hereinafter, in thelow-speed range and as the pressure proportional to the vehicle speed orthe governor pressure decreases substantially, a flow circuit inparallel with line 112 is formed by line 113, branch valve 64, and line114. The governor pressure is applied to a chamber 231 of branch valve64.

The control pressure in line 103 is applied to shift valve 5 and then toa line 121 through a chamber 242, if the governor pressure increasesabove a predetermined value for the pressure then proportional to thethrottle valve opening, or the throttle pressure. Thus, the pressure ofline 103 is applied to clutch 8 and to a chamber 222 of the servo forbrake band 21, to engage clutch 8 and to release brake band 21simultaneously. Thereby, the second speed or the higher speed isestablished.

On the contrary, if the governor pressure decreases below apredetermined value for the particular throttle responsive pressure,shift valve 59 operates to shut off the control pressure from line 103,and, at the same time, pressure oil forced into clutch 8 and into thebrake band servo chamber 222 is discharged into chamber 242 through line121 and flows to exhaust port 244. Thus, the transmission is restoredagain to the first or lower speed.

The governor pressure acts on a chamber 243 at the right end of shiftvalve 59, and the throttle pressure acts on a chamber 241 containing aspring 59b. The governor pressure, obtained by action of governor 53, isavailable in line 106 which branches from the discharge line or rear oilpump 52.

The throttle pressure operates to transmit the movement of the enginethrottle valve or accelerator (not shown) to plug 58b in throttle valve58, and urges valve body 58a to the right through the medium of spring58c. Thus the control pressure of line 101 is applied to chamber 251,from which it is applied to line 104 as a pressure proportional to theopening of the engine throttle valve.

A line 108 connects a chamber 205 of pressure regulator valve 54 to achamber 213 of the manually operated valve 57. The purpose of this lineis to supply the control pressure in chamber 211 of manual valve 57through line 112 to chamber 213, and also to the chamber 205 of pressureregulator valve 54, when manual valve 57 is moved to the L-position.Through line 108, when valve 57 is in the L-position, the control pressure in 205 urges valve body 54a to the left in conjunction with spring54!: of pressure regulator valve 54, so that the control pressure inchamber 203 is constantly controlled at a higher pressure. Namely, inthe low-range L, the brake band servo only is operated as in the firstor low speed of the D range, and naturally a large torque is required inthe transmission, so that a particularly high control pressure must besupplied not only to the servo for brake band 21 but also to torqueconverter C.

As explained above, there is a difference in the magnitude of thecontrol pressure, between the drive range D and the low-range L.However, each of these pressures is always controlled to be constantindependently of the transmission torque in the transmission gear. Insuch a hydraulic pressure control system, when a particularly largetorque is required, the control pressure to actuate clutch 3 and brake22 and brake band 21 becomes insufficient. In order to prevent this, thecontrol pressure must previously be set to a higher value in acorresponding degree beforehand.

For this purpose, in accordance with the present invention, there isprovided a pressure modulator valve 61 which applies a compensatingpressure through a line 107 to a chamber 201 at the left hand of thevalve body 540 in pressure regulator valve 54, the control pressure inchamber 203 being controlled or regulated by this compensating pressure.The compensating pressure is intended to have such characteristics thatthe control pressure is proportional to the torque transmitted throughthe transmission at the time of control. FIG. 3 illustrates thehydraulic circuit diagram for pressure regulator valve 54 with pressuremodulator valve 61, in more detail than illustrated in FIG. 2.

Referring to FIG. 3, pressure modulator valve 61 includes a movablevalve body 61, a movable valve plug 61b, which is movable independentlyof valve body 61a, and a compression spring 610 interposed between valvebody 61a and plug 61b. Valve 61 also includes chambers 261, 262, 264,266 and 267, valve port 263 and oil discharge port 265 between the endsof a cylindrical valve chamber. A stop plug 61d closes the right end ofthe valve chamber. Chamber 261 is connected to line 104 by means ofwhich it is supplied with a pressure proportional to the opening of thethrottle valve, or the throttle pressure from the throttle valve. Valveport 263 is connected to control pressure line 101.

By movement of valve body 6111, with resulting change in the flowcross-sectional area of chamber 264, the control pressure in line 101 isconverted into a compensating pressure having characteristics, asexplained hereinafter, to be supplied from chamber 264 to line 107. Thecompensating pressure, also effective in chamber 262 through orificesand the like, will act as an operating pressure to urge valve body 61atoward the right in conjunction with the throttle pressure in line 104.Discharge port 265 communicates with chamber 264 upon movement to theright of valve body 61a and, upon blocking of valve port 263, thepressure in chamber 264 is reduced to zero. The control pressure in line101 is applied to chamber 266 of valve 61 to urge plug 61b to the right.How ever, in chamber 267 there is a pressure proportional to thegovernor pressure, supplied from line 106, which urges movable plug 61bto the left. Thus, the operating force exerted by the control pressurein chamber 266, and the operating force exerted by the control governorpressure in chamber 267, are in opposition, so that a difference betweenthese pressures is necessary to operate movable plug 61b. When theoperating force is operable to move plug 61b to the right, or weakerthan the rightward operating force acting on valve body 61a, even in theleft direction, the movable plug 61b is moved to the right to engagestop plug 61, where plug 61b is restrained by an abutment or projectionon its end face so as to maintain a space between itself and plug 61d.

Under these conditions, movable plug 61b is restrained from furthermovement to the right, and the operating forces applied to plug 61b bythe pressure in chambers 266 and 276 have no significance for valve body61a, so that movable plug 61b serves only as a restraining member forone end of spring 610. In this case, the compensating pressure will becontrolled solely in dependence upon the throttle pressure.

With an increase of the throttle pressure acting in chamber 261, valvebody 61a is moved to the right to narrow valve port 263 to lower thepressure in chamber 264 and simultaneously to lower the pressure inchamber 262. This pressure control is effected utilizing extremely smallflow sectional areas of valve port 263 and chamber 264 so that, althoughthere is a comparatively large increase in the throttle pressure,movement of valve body 61a remains small and the change in thecompression of spring 61c, due to the movement to the right of valvebody 61a, is also small. Thus, even though the throttle pressureincreases, the force of spring 61c is maintained nearly constant. Also,the force acting on valve body 61a and opposing this nearly constantforce of spring 610 must be constant, so that a resultant of theoperating force due to the throttle pressure in chamber 261 and theoperating force due to the compensating pressure in chamber 262 alsoshould be nearly constant.

To meet this requirement, the compensating pressure should have acharacteristic which decreases rectilinearly in accordance with anincrease of the throttle pressure, and such a relation is shown in FIG.4. In FIG. 4, the curve Pth represents the change of throttle pressureobtainable in throttle valve 58 responsive to opening of the enginethrottle. This throttle pressure does not increase proportionally to theopening of the throttle, but remains constant in a small opening rangeand also in a range near to the full opening of the throttle. This isthe reason for the design of the throttle valve 58.

On the other hand, the compensating pressure shown by the curve Pthm isalso maintained constant and has a high value with a small throttleopening range and at the constant throttle pressure Pth, while it isdecreased rectilinearly to eventually reach zero in accordance with anincrease of the throttle pressure Pth. When the value reaches zero, theoperating force due to the throttle pressure as applied to valve body61a overcomes the force of spring 61c, which exhibits a constantreaction maintaining a constant compression value. By virtue of thisrelation, the resultant, that the operating force due to the throttlepressure in chamber 261 and the operating force due to the compensatingpressure in chamber 262, is constant, will no longer be true, and valvebody 61a fully blocks valve port 263 to connect chamber 264 to dischargeport 265, thus maintaining the compensating pressure at zero. Thethrottle opening at which the compensating pressure reaches zero can beregulated by changing the characteristic of spring 61c.

The compensating pressure as shown by curve Pthm in FIG. 4, is appliedto chamber 201 of pressure regulator valve 54 through line 107, to actas an operating force to urge valve body 54a to the right in cooperationwith the control pressure in chamber 202. The regulation of the controlpressure in line 101, as previously explained with reference to FIG. 2,is effected in a manner such as to discharge the control pressure oil tooil sump 50a through chamber 204 by movement to the right of valve body54a. The resultant of the operating force due to the compensatingpressure and the operating force due to the control pressure, on valvebody 540, should be a nearly constant value corresponding to the forceof spring 54b similarly to the conditions in pressure modulator valve61.

As a consequence, the control pressure decreases rectilinearly with anincrease of the compensating pressure. In other words, it increasesrectilinearly with a decrease in the compensating pressure. The curve PLof FIG. 4 illustrates the control pressure as regulated in this manner.As clear from this curve, the control pressure is regulated to aconstant value with a relatively small throttle opening range. When theopening of the throttle valve exceeds this small range, so as to lowerthe compensating pressure Pthm to zero, the control pressure increasesto a constant maximum value which it does not exceed. At less than thisthrottle valve opening, control pressure P, varies in accordance withthe throttle valve opening, so that there is obtainable a controlpressure in the transmission corresponding substantially to the torquedemand. Therefore, inadequate control pressure for operation of theservos can be prevented perfectly as, for example, when the vehicle isproceeding upgrade or is starting on an upgrade.

In pressure modulator valve 61 if, with an increase of the governorpressure acting in chamber 267, the operating force tending to move plug61b to the left exceeds the operating force tending to move valve body61a to the right, movable plug 61b, which so far has been moved to theright and restrained, produces a net force to move valve body 61a to theleft through the medium of spring 61c. This force, as will be understoodfrom the foregoing explanation, acts to increase the compensatingpressure and, moreover, decrease the control pressure in pressureregulator valve 54. This decrease of the control pressure results in adecrease of the counter operating force in chamber 266 of pressuremodulator valve 61, so that the force exerted by movable plug 61b tomove valve body 61a to the left is greatly increased to move valve body61a suddenly to the left to increase the opening of valve port 263, thusresulting in equalizing the control pressure and the compensatingpressure. Namely, under such conditions, not only the throttle pressurebut also the governor pressure become no longer related to the magnitudeof the compensating pressure. If a compensating pressure equal to thiscontrol pressure is applied to the chamber 201 of pressure regulatorvalve 54, only the control pressure, together with the control pressureof chamber 202, acts as an operating force to urge valve body 540 to theright, and is kept at a constant and low value corresponding to thespring 54b.

This condition is illustrated in FIG. 5. The horizontal lines a, througha illustrate that each of the control pressures is main tained constantcorresponding to the opening of the engine throttle, on the curve P andindependently of the vehicle speed or the governor pressure. That is,the group of horizontal lines a through 0,, illustrate control pressurescorresponding to engine throttle openings established at a vehicle speedrange in the low-speed area.

Line a represents a characteristic when the throttle is wholly closed.Each of the lines a: through 11., represents a characteristiccorresponding to gradual increase of the throttle opening. Line (1represents a characteristic when the control pressure is limited to themaximum value, between a certain throttle opening and a full throttleopening.

Thus, when the vehicle speed gradually rises in accordance withrespective throttle valve openings and, if the speed reaches a certainvalue corresponding to the respective throttle valve opening, each ofthe control pressures decreases suddenly as illustrated by the verticallines b, through 27,, and then is maintained at a constant and low valueas shown by the horizontal line C. The governor pressure, as explainedabove, attains a value, at a speed higher than that corresponding to thevertical line 11,, which is a borderline for changing over to fullopening of the throttle, such that, under such conditions, acompensating pressure equivalent to the control pressure is continuouslyapplied to pressure regulator valve 54 from pres sure modulator valve61, and thereafter a constant control pressure, such as normallyrequired is supplied to the hydraulic servos.

In accordance with the present invention, at the low-speed range Lrequiring, comparatively, a large accelerating torque, a controlpressure corresponding to the throttle opening is obtainable, so that acontrol pressure sufficiently high can be supplied to the hydraulicservos for operation of the clutches and brake bands of thetransmission. This prevents inadequacy of the servo capacity at the timeof ascending an upgrade or starting the vehicle on an upgrade. As acontrol pressure is maintained at the low pressure usually required whenthe vehicle speed exceeds a certain value, in the case of the automaticgear change being at the D position, gear shifting can be effectedsmoothly without disagreeable impact and noise. Moreover, in the rangewhere the control pressure is controlled according to the throttleopening, the control pressure is limited to a maximum value between acertain opening of the throttle, less than full opening, and fullopening of the throttle, so that the conduit system can be adequatelyprotected against excessive pressure.

Additionally, while the foregoing explanation has been directed to thecondition where manual valve 57 is in the D position, even if thevehicle is driven at low speed with the manual valve 57in the Lposition, the control pressure is controlled by pressure modulator valve61, to assure proper operation of the servos.

The case is similar with respect to reverse" movement of the vehicle,with valve 57 in the R position. Additionally, it should be understoodthat the invention is applicable not only to automatic transmission ofthe type having two forward speeds and one reverse speed, as explainedas a preferred embodiment, but also to transmissions having a differentnumber of forward and reverse speeds.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedwithout departing from such principles.

What is claimed is:

1. In an automatic transmission for a motor vehicle having an enginedrive shaft, a manually operable throttle and a manually operable speedselector, the transmission including a hydraulic servo-operated speedchange gear, fluid torque transmitting means coupling the engine driveshaft to the speed change gear, a hydraulic pressure regulating valveregulating a control hydraulic pressure supplied to the servooperatedspeed change gear, and a speed responsive governor: a hydraulic pressurecontrol system comprising, in combination, a pressure modulator valveincluding a movable valve body and a movable plug movable independentlyof said valve body, said plug being operable to apply an operating forceto I said movable valve body; means connecting said pressure modulatorvalve to said pressure regulating valve to apply a compensatinghydraulic pressure to said pressure regulating valve in accordance withdisplacement of said valve body; a throttle valve operable bysaidthrottle and connected to said pressure modulator valve to apply, tosaid movable valve body, a hydraulic pressure proportional to opening ofsaid throttle to vary said compensating pressure in accordance with theopening of said throttle to correspondingly vary said control hydraulicpressure; and means, including said governor, applying directly to oneside of said movable plug only a hydraulic pressure proportionaldirectly to the vehicle speed and, to the other side thereof, said onlycontrol hydraulic pressure; whereby said movable plug exerts nooperating force on said valve body until the hydraulic pressureproportional to the vehicle speed exceeds a predetermined value; saidmovable plug, when the hydraulic pressure proportional to the vehiclespeed exceeds said predetermined value, exerting an operating force onsaid valve body in opposition to said hydraulic pressure proportional tosaid throttle opening, to move said valve body in a direction to apply,to said pressure regulating valve, a compensating pressure independentof the throttle opening to maintain said control hydraulic pressureconstant.

2. In an automatic transmission for a motor vehicle, a hydraulicpressure control system, as claimed in claim 1, in which, when thethrottle opening exceeds the predetermined value while the hydraulicpressure proportional to the vehicle speed is below a predeterminedvalue, said modulator valve applies, to said pressure regulating valve,a compensating pressure independent of the throttle opening andeffective to maintain said control hydraulic pressure at a constantmaximum value.

3. In an automatic transmission for a motor vehicle, a hydraulicpressure control system, as claimed in claim 1, in

which said bore includes a chamber connected to said pressure regulatingvalve and having said control hydraulic pressure supplied theretothrough a port; said movable valve body, responsive to a hydraulicpressure proportional to opening of said throttle and above apredetermined value, blocking said port and connecting said chamber to adischarge port to reduce said compensating pressure to zero.

l t #i i

1. In an automatic transmission for a motor vehicle having an enginedrive shaft, a manually operable throttle and a manually operable speedselector, the transmission including a hydraulic servo-operated speedchange gear, fluid torque transmitting means coupling the engine driveshaft to the speed change gear, a hydraulic pressure regulating valveregulating a control hydraulic pressure supplied to the servo-operatedspeed change gear, and a speed responsive governor: a hydraulic pressurecontrol system comprising, in combination, a pressure modulator valveincluding a movable valve body and a movable plug movable independentlyof said valve body, said plug being operable to apply an operating forceto said movable valve body; means connecting said pressure modulatorvalve to said pressure regulating valve to apply a compensatinghydraulic pressure to said pressure regulating valve in accordance withdisplacement of said valve body; a throttle valve operable by saidthrottle and connected to said pressure modulator valve to apply, tosaid movable valve body, a hydraulic pressure proportional to opening ofsaid throttle to vary said compensating pressure in accordance with theopening of said throttle to correspondingly vary said control hydraulicpressure; and means, including said governor, applying directly to oneside of said movable plug only a hydraulic pressure proportionaldirectly to the vehicle speed and, to the other side thereof, said onlycontrol hydraulic pressure; whereby said movable plug exerts nooperating force on said valve body until the hydraulic pressureproportional to the vehicle speed exceeds a predetermined value; saidmovable plug, when the hydraulic pressure proportional to the vehiclespeed exceeds said predetermined value, exerting an operating force onsaid valve body in opposition to said hydraulic pressure proportional tosaid throttle opening, to move said valve body in a direction to apply,to said pressure regulating valve, a compensating pressure independentof the throttle opening to maintain said control hydraulic pressureconstant.
 2. In an automatic transmission for a motor vehicle, ahydraulic pressure control system, as claimed in claim 1, in which, whenthe throttle opening exceeds the predetermined value while the hydraulicpressure proportional to the vehicle speed is below a predeterminedvalue, said modulator valve applies, to said pressure regulating valve,a compensating pressure independent of the throttle opening andeffective to maintain said control hydraulic pressure at a constantmaximum value.
 3. In an automatic transmission for a motor vehicle, ahydraulic pressure control system, as claimed in claim 1, in which saidpressure modulator valve includes a bore in which said movable valvebody and said movable plug are movable; a spring interposed between saidmovable plug and one end of said movable valve body; and means applying,to the opposite end of said movable valve body, a hydraulic pressurepropoRtional to opening of said throttle.
 4. In an automatictransmission for a motor vehicle, a hydraulic pressure control system,as claimed in claim 3, in which said bore includes a chamber connectedto said pressure regulating valve and having said control hydraulicpressure supplied thereto through a port; said movable valve body,responsive to a hydraulic pressure proportional to opening of saidthrottle and above a predetermined value, blocking said port andconnecting said chamber to a discharge port to reduce said compensatingpressure to zero.